1. Field of the Invention
This invention relates to a method and device for controlling the tool electrode in, for example, an electrical discharge machine tool used for machining workpieces such as steel workpieces; more specifically, to a control device which, when the waveform of the electrical discharge between a tool electrode and workpiece becomes abnormal, is capable of rapidly eliminating the abnormality.
2. Description of the Related Art
Consider, for example, an electrical discharge machine tool which machines workpieces such as steel workpieces. In general, the tool electrode and the workpiece are placed opposite each other, separated by a very small gap, in an electrically insulating dielectric fluid such as kerosene. Voltage pulses are repeatedly applied between the tool electrode and workpiece to machine the workpiece by melting and removing metal from its surface. In this type of electrical discharge machine tool, electrical discharges repeat at the pulse frequency and the tool electrode is gradually submerged inside the workpiece. At this time, if the gap between the tool electrode and the workpiece is too large, an electrical discharge will not take place. Conversely, if the gap is too small, a short circuit will occur. Therefore, the electrical discharge machine tool contains a servomechanism which accurately controls the tool electrode position to insure that the correct gap is maintained between the tool electrode and the workpiece.
In an electrical discharge machine tool as described above, when fine metal waste products removed from the surface of the workpiece by the machining and products of decomposition of the dielectric fluid are present between the tool electrode and the workpiece, the electrical insulation between the tool electrode and the workpiece is reduced and normal discharges do not take place, causing loss of machining accuracy. Consequently, various methods are employed to remove metal waste products from between the tool electrode and the workpiece. For example, when it is possible to open a hole for the dielectric fluid in the tool electrode or the workpiece, dielectric fluid can be ejected or sucked out through the hole. However, when it is not possible to open a hole for the dielectric fluid in the tool electrode or the workpiece, it is necessary to use another method such as strongly injecting the dielectric fluid into the gap between the tool electrode and the workpiece or periodically moving the tool electrode up and down to create a pumping action to remove waste products of the machining from between the tool electrode and the workpiece.
When machining the workpiece to form a dead-end hole such as a hole with a bottom, use of the injection and pumping actions together is effective in removing waste products of machining from the dead-end hole. However, in the method which involves periodically moving the tool electrode up and down, even if the waveform of the electrical discharge between the tool electrode and the workpiece is normal, the need to move the tool electrode up and down unavoidably causes a drop of machining efficiency.
In addition, in an electrical discharge machine tool as described above, the unit distance which the tool electrode is moved in a certain time is kept to a relatively small value to keep the machining process stable, and the motion is slow. Consequently, if, for example, waste products of machining cause a short circuit between the tool electrode and the inside of a deep hole, the tool electrode cannot be pulled away at a high speed, so that a considerable time is required before the short circuit is eliminated. Further, when, as described above, the tool electrode must be pulled back a relatively large distance, considerable additional time is required before the tool electrode can be returned to its original position.
In addition, in an electrical discharge machine tool such as described above, sometimes three-dimensional machining is performed by moving the tool electrode relatively in the X, Y and Z directions with respect to the workpiece. In this type of machining, the tool electrode is controlled based on the main program pre-stored in a computer control unit which controls the electrical discharge machine tool. Since it is not easy to control the tool electrode through a path that differs from the path in the program while machining is in progress, when it becomes necessary to remove waste products of machining from between the tool electrode and the workpiece or to eliminate a short circuit, all that is done is to move the tool electrode up and down, so that effective pump action cannot be expected in a case such as machining a big hole.